final dio = Dio();

Dio getMyDio() {
initAdapter();
dio.options.headers = {'apiKey': 'xxxxx'};
dio.options.connectTimeout = const Duration(seconds: 15);
dio.options.receiveTimeout = const Duration(seconds: 15);
dio.options.validateStatus = (_) => true;
return dio;
}

void initAdapter() {
dio.httpClientAdapter = IOHttpClientAdapter(
onHttpClientCreate:(client) {
final HttpClient client = HttpClient(context: SecurityContext(withTrustedRoots: false));
client.badCertificateCallback = (cert, host, port) => true;
return client;
},
);
}

用flutter Dio实现适配器属性的自定义

用类似的配置，给dio生成自定义的httpClient，效果类似Android入门教程 | OkHttp + Retrofit 使用 - 掘金 (juejin.cn)中使用retrofit和okhttp设置自己的httpClient并信任所有证书。